1. Field of the Invention
The present invention relates to a development apparatus used in electrostatographic printing machines, and more specifically, the present invention relates to a particular material composition for a ceramic coated donor roll for use in a hybrid scavengeless or hybrid jumping development apparatus.
2. Description of the Prior Art
Generally, the process of electrostatographic reproduction includes uniformly charging a photoconductive member, or photoreceptor, to a substantially uniform potential, and then imagewise discharging it or imagewise exposing it to light reflected from an original image that is being reproduced. The result is an electrostatically formed latent image on the photoconductive member. The latent image that is formed is developed by bringing a charged developer material into contact with the latent image. Two component and single-component developer materials are commonly used. A typical two-component developer material comprises magnetic carrier particles, having charged toner particles adhering triboelectrically thereto. A single component developer material typically comprises charged toner particles only. In either case, the charged toner particles when brought into contact with the latent image, are attracted to such image, thus forming a toner image on the photoconductive member. The toner image is subsequently transferred to a receiver sheet which is then passed through a fuser apparatus where the toner image is heated and permanently fused to the sheet, thus forming a hard copy of the original image.
To develop a latent image in an electrostatographic reproduction machine as described above, charged toner particles either alone (single component), or mixed (two-component), are brought, by a development apparatus, into contact with the latent image formed on the photoreceptor. For two-component development, developer material containing carrier particles and toner particles is used. The development apparatus for such development typically includes a housing defining a chamber within which the developer material is mixed and charged. Moving and mixing two-component developer material triboelectrically and oppositely charges the carrier particles and the toner particles causing the toner particles to adhere to the carrier particles.
As disclosed for example in U.S. Pat. No. 5,245,392, and U.S. Ser. No. 07/091,858 both assigned to the assignee of the present application, one type of a two-component development method and apparatus is referred to as xe2x80x9chybrid scavengeless developmentxe2x80x9d, and is very suitable for image-on-image development type processes. The apparatus includes a housing defining a development zone, and a mixing chamber holding developer material containing carrier and toner particles. The apparatus also includes a magnetic roll and a donor member such as a donor roll for receiving charged toner particles from the magnetic roll and transporting them to the development zone. A plurality of electrode wires are embedded in, or are closely spaced relative to, the donor roll within the development zone. An AC voltage is applied to the electrode wires for forming a toner cloud in the development zone. Electrostatic fields generated by an adjacent latent image on a photoreceptor surface serve to attract charged toner particles from the toner cloud, thus developing the latent image.
Single component development systems, referred to as jumping gap development, can also use a donor roll for transporting charged toner particles directly from a toner chamber to the development zone. The charged toner particles similarly are attracted by and develop an electrostatic latent image recorded on a photoconductive surface. In jumping gap development, an AC voltage is applied to the donor roll for detaching toner particles from the donor roll and projecting them toward an adjacent photoconductive surface holding the electrostatic latent image.
In either of the above discussed development systems for example, the donor member or roll and its electrical and chemical characteristics are very important to the ability of the development apparatus repeatably transport acceptable and uniform quantities of toner particles into the development zone, as well as effectively support the electrostatic fields necessary within the development zone for high quality image development. For example, the donor roll must be suitable for charged toner particles to effectively and controllably (even at high speeds) adhere electrostatically thereto. The surface of the donor roll must be partially conductive relative to a more conductive core, and this partial conductivity on the surface should be uniform throughout the entire circumferential surface area. The range of conductivity of a donor roll should be well chosen in order to maximize the efficiency of a donor roll in view of any number of designed parameters, such as energy consumption, mechanical control and the discharge time-constant of the surface thereof.
In image-on-image type processes with a pre-developed toner image already on the photoreceptor, the donor roll should also act as an electrostatic xe2x80x9cintermediatexe2x80x9d between the photoreceptor and the developer transport roll in order to minimize unwanted interactions between the development system and the photoreceptor. Minimizing such interactions is particularly desirable in such processes because the single photoreceptor therein is to be charged, exposed and developed several times usually in a single, as in single pass highlight color process or in a single pass color process.
The donor roll must further have desirable wear-resistant properties so that the surface thereof will not be readily abraded by adjacent surfaces. Further, the surface of the donor should be without anomalies such as pin holes, which may be created in the course of its manufacture. Pinholes created in the manufacturing process or abrasions caused in its use, can result in electrostatic xe2x80x9chot spotsxe2x80x9d and undesirable electrical arcing in the vicinity of such structural imperfections. Ultimately, the most important requirement of the donor roll can be summarized by the phrase xe2x80x9cuniform conductivity;xe2x80x9d Other physical properties of the donor/roll, such as the mechanical adhesion of toner particles, are also important, but are generally not as quantifiable in designing development apparatus.
Known coating materials for donor rolls basically consist of a mechanical blend of two different starting ceramic powders, each consisting of varying levels of, for example, alumina and titania. These two starting powders are mechanically blended in a specific ratio to achieve the desired percent of alumina and titania. This process requires weighing the two starting powders to achieve the correct ratio, and then blending the two powders together to achieve a homogenous mixture. An error in weighing results in a donor roll coating that does not meet its electrical property specification.
Examples of these alumina-titania coating material blends are described in U.S. Pat. Nos. 5,473,418 and 5,600,414.
In accordance with features of the disclosed embodiments presented herein, a toner donor roll for use in a development apparatus, comprises a conductive core; and a ceramic outer coating over the conductive core, the ceramic coating being formed from thermal spraying a single homogeneous powder consisting of particles each of which contains a specific ratio of pure alumina and pure titania held together with an organic binder.
In accordance with still other features of the disclosed embodiments described herein an apparatus for developing a latent electrostatic image on a surface comprises a housing defining a chamber storing developer material containing toner particles; means mounted partially within the chamber for moving the developer material; and at least one rotatable donor roll for transporting toner particles into a development transfer relationship with the latent electrostatic image on the surface, the donor roll being mounted in a toner particle receiving relationship with the developer material moving means, the donor roll including a core, and a ceramic outer coating, the ceramic coating formed from thermal spraying a single homogeneous powder consisting of particles each of which contains a specific ratio of pure alumina and pure titania held together with an organic binder.
In accordance with still other features of the disclosed embodiments a printing machine comprises an image bearing surface; means for electrostatically forming a latent image on the image bearing surface; and a development apparatus for developing the latent electrostatic image, the development apparatus including: a housing defining a chamber storing developer material containing toner particles; means mounted partially within said chamber for moving the developer material; and at least one rotatable donor roll for transporting toner particles into a development transfer relationship with the latent electrostatic image on the image bearing surface, the donor roll being mounted in a toner particle receiving relationship with the developer material moving means, the donor roll including a core, and a ceramic outer coating, the ceramic outer coating formed from thermal spraying a single homogeneous powder consisting of particles each of which contains a specific ratio of pure alumina and pure titania held together with an organic binder.